This invention relates to a method for manufacturing variable capacitance pressure transducers and to an intermediate article of manufacture produced in the practice of the method. The pressure transducers are suitable for use in sensing either absolute or gage pressures or pressure differentials. The intermediate article of manufacture comprises an aggregation of pressure-sensitive electrical capacitors which may be separated to provide individual paired or otherwise grouped pressure transducers.
Recently, motor vehicle applications have required the use of pressure sensors for measuring both ambient and subatmospheric pressure levels. In internal combustion engine applications, fine control of fuel metering has required that the rapidly-fluctuating pressure levels within the intake manifold of the engine be measured as well as the less-rapidly fluctuating ambient pressure levels. Sensors able to measure these pressures reliably and with adequate response time have been difficult to obtain and are very expensive. Perhaps the best pressure sensor thus far available is of the ceramic pressure transducer type. This variable capacitance sensor has a thick ceramic element on which is deposited a metal film. A second and very thin ceramic plate or diaphragm also has a metal film deposited on it and is attached with a glass frit or the like to the thicker ceramic element. The metal films on the ceramic element constitute the plates of a capacitor. The thin ceramic diaphragm responds to pressure variations to produce a small change in capacitance that is used to vary the frequency produced by an electronic oscillator.
Sensors or transducers of the type described in the preceding paragraph are expensive because of their large size and because they must be manufactured individually. Also, a ceramic pressure transducer of this type may not be tested until it has been assembled with its associated electronic circuitry. If it is found to be defective, the entire assembly then must be discarded.
A pressure transducer that could be manufactured by batch-processing techniques is a desirable device. Also desired is a pressure transducer that is of small size and that can be tested prior to assembly with electronic circuitry that may be associated with the device to produce a signal proportional to the pressure detected by the transducer element. The pressure transducer should be inexpensive with respect to the materials utilized, should be reliable and should be inexpensive as compared to prior art devices. Moreover, the pressure sensor should be easily calibrated, should be mass producible at low cost and should provide an accurate and reproducible variable parameter in response to fluctuations in the pressure to be sensed. Desirably, the pressure transducer should be manufacturable for use over various pressure ranges.
The method of the present invention and the intermediate article of manufacture produced thereby may be used in the manufacture of pressure transducers having rapid response times and to provide the desired characteristics mentioned above.